TOKYO, May 14 (Reuters) - Would Japan send its military to defend the Philippines if it was attacked by China? That's the kind of question Prime Minister Shinzo Abe could well face as he pushes for a landmark change to security policy.

Abe's private advisers will on Thursday present him with a report urging a loosening of legal limits on Japan's military, including an end to a decades-old ban on helping allies under attack that has kept Japanese forces from fighting abroad since World War Two.

The report, a draft of which was obtained by Reuters, sets the stage for Abe to make a pitch for a historic change in a defence policy that has long been based on the principle that Japan has the right to defend itself with the minimum necessary force, but that combat abroad exceeds the limit.

A lifting of the ban on "collective self-defence" would be welcome to Japan's ally the United States, but would draw criticism from China, ties with which have been damaged by a territorial row and the legacy of Japan's past aggression.

Despite Abe's desire to loosen the limits of the U.S.-drafted charter, doubts remain about how far and how quickly he can go. His Liberal Democratic Party's (LDP) junior partner, the New Komeito, is wary, voters are divided and the LDP's deputy leader is worried about the impact on local polls this year and next.

"This is not a simple matter that can just be pushed through," a senior LDP lawmaker told Reuters. "It's complex."

Previous governments have said Japan has the right of collective self-defence under international law, but that the constitution's pacifist Article 9 prohibited taking such action.

Abe's advisers argue that Japan's security environment, including an increasingly assertive China and volatile North Korea, requires a more flexible approach.

Tension in the region grew this month when China positioned an oil rig in an area of the South China Sea also claimed by Vietnam and spiked again when China demanded that the Philippines release a Chinese fishing boat seized off Half Moon Shoal in the Spratly Islands, claimed by both countries.

Japan itself is locked in a feud with China over Japanese-controlled isles in the East China Sea.

"Looking at China's moves, everyone thinks that in broad terms, allowing the exercise of the right of collective self-defence can't be avoided. But when the debate gets specific, it gets difficult. The question is, what kind of limits should be set?" said a second senior LDP lawmaker who also declined to be identified.

"Could we say a clash between the Philippines and China has nothing to do with Japan? Some would say after the South China Sea comes the East China Sea. Others would say helping the Philippines is going too far," he told Reuters.

ACT WHILE SUPPORT RATES HIGH

Given domestic and diplomatic dynamics, a blanket lifting of the ban soon appears unlikely - although critics say even small changes would open the door to more drastic moves later.

"There are some changes to which the New Komeito is strongly opposed," the first LDP lawmaker said.

Critics also say Abe is taking a stealthy approach by seeking to reinterpret the constitution instead of the more politically difficult step of formally revising the U.S.-drafted charter, which has never been altered since its adoption in 1947.

Abe would like to embody the change in a cabinet resolution next month, before the parliamentary session ends on June 22, or at the latest by September, to leave time to revise a slew of related laws in an extra session of parliament late in the year.

"The prime minister wants to act while his support rates are high and politics stable," the first LDP lawmaker said.

Supporters of the change also want it settled in time to be reflected in updated U.S.-Japan defence cooperation guidelines that Japan wants to finish by year-end.

Abe will unveil the government's response to the report on Thursday, outlining cases in which legal limits on the military could be loosened as a basis for talks inside the ruling bloc.

Examples in the report include protecting a U.S. warship under attack in waters near Japan; mine-sweeping in sea-lanes in a conflict zone; intercepting a ballistic missile headed for America; and inspecting vessels supplying arms to a country that has attacked the United States.

The advisers will also state there are no constitutional constraints on Japan's participation in U.N.-led collective security operations, in which nations join to repel an aggressor against one state, but the government is unlikely to push for that even more controversial change, political sources said.

The draft also recommends legal changes to facilitate action not directly tied to collective self-defence but where the military has to date been constrained by legal concerns, such as rescuing Japanese overseas, using weapons in U.N. peace-keeping operations and dispatching troops to low-intensity conflicts that fall short of a full-scale attack on Japan. (Editing by Robert Birsel)